Humans are largely diurnal organisms and have biological clocks that are, usually and at least roughly, set to the dark/light cycles caused by the rotation of the earth around its own axis. In premodern times, the time of day was kept as "solar" time; the end of each day, and the beginning of the next, were tied to when the sun came up and went down at that location on the earth. Twelve O'clock midnight in one location differed by a few seconds from twelve o'clock midnight a few miles away east or west. The change of the time as a function of change of longitude was not a problem before air travel became possible; with even the fastest premodern methods of travel, the traveler's biological clock easily adjusted to whatever time was extant at the location at which he was arriving.
It was only with the advent of intercontinental commercial aviation in the last half of the twentieth century that travel became so rapid that the body's biological clock could no longer easily adjust. "Jet lag" became a phenomenon. Conventionally, the intercontinental traveler is faced with two choices: attempt to function at the new location with a sleep deficit, usually at reduced efficiency; or spend one or two additional days resting and adjusting to the time of day at the new location. Then, at the terminus of the traveler's visit, jet lag is experienced in the other direction, and is dealt with in much the same way. This traditional coping mechanism is very wasteful of a traveler's time and is an impediment to productivity. To a somewhat lesser extent, the same situation is true for travel within a continent, depending on how far the trip extends, e.g., coast-to-coast travel within the U.S. mainland.
Recently, research has shown that the biological clock may be readjusted by shining a light on the popliteal region of the body, located at the back of a patient's knees. Oren, D. and Terman, M., "Tweaking the Human Circadian Clock with Light", Science 279: 333-334 (January 1998); Campbell, S, and Murphy, P., "Extraocular Circadian Phototransduction in Humans", Science 279: 396-399 (January 1998); Hopkin, K., "Clock Setting", Scientific American, vol. 278, no. 4, pp. 20-22 (April 1998). This finding was surprising because up until this time interaction with daylight conditions was believed to be only through the eyes. But because the back of the knee, or popliteal region, has many blood vessels relatively close to the surface, it can serve as a location at which light may interact with photoactive substances in the blood. A New York Times article reporting this finding (Jan. 16, 1998) suggested that travelers from New York to Paris could wear knee braces with light sources which would reset their biological clock as they slept during the flight. Still, this solution is cumbersome, adding to cabin clutter and requiring installation and removal at the beginning and end of the flight, and the inventors have come up with apparatus which will more easily accomplish the same objective.